Fix the configuration of the RomFS to make sure that at least one 
backing store method is always selected.  This is done by rendering it 
down to a choice item that selects between Block, MTD and both.

This also works correctly in the case that CONFIG_MTD=m: MTD cannot be 
selected as a backing store unless CONFIG_ROMFS_FS is also 'm'.
Signed-off-by: NDavid Howells <dhowells@redhat.com>
Signed-off-by: NDavid Woodhouse <David.Woodhouse@intel.com>

Change RomFS so that it can use MTD devices directly - without the intercession
of the block layer - as well as using block devices.

This permits RomFS:

 (1) to use the MTD direct mapping facility available under NOMMU conditions if
     the underlying device is directly accessible by the CPU (including XIP);

 (2) and thus to be used when the block layer is disabled.

RomFS can be configured with support just for MTD devices, just for Block
devices or for both.  If RomFS is configured for both, then it will treat
mtdblock device files as MTD backing stores, not block layer backing stores.

I tested this using a CONFIG_MMU=n CONFIG_BLOCK=n kernel running on my FRV
board with a RomFS image installed on the mtdram test device.  I see my test
program being run XIP:

	# cat /proc/maps
	...
	c0c000b0-c0c01f8c r-xs 00000000 1f:00 144        /mnt/doshm
	...

GDB on the kernel can be used to show that these addresses are within the
set-aside RAM space.
Signed-off-by: NDavid Howells <dhowells@redhat.com>
Tested-by: NBernd Schmidt <bernd.schmidt@analog.com>
Signed-off-by: NDavid Woodhouse <David.Woodhouse@intel.com>

Signed-off-by: NAlexey Dobriyan <adobriyan@gmail.com>